Merge #10680

10680: implement Literal::from_str r=jonas-schievink a=spookyvision

this apparently fixes RTIC 0.6's 
```rust
#[rtic::app]
mod app {}
```

Co-authored-by: Anatol Ulrich <anatol.ulrich@ferrous-systems.com>

Author: bors[bot]

crates/proc_macro_srv/src/abis/abi_1_55/rustc_server.rs

@@ -488,8 +488,8 @@ impl server::Literal for Rustc {
         // They must still be present to be ABI-compatible and work with upstream proc_macro.
         "".to_owned()
     }
-    fn from_str(&mut self, _s: &str) -> Result<Self::Literal, ()> {
-        unimplemented!()
+    fn from_str(&mut self, s: &str) -> Result<Self::Literal, ()> {
+        Ok(Literal { text: s.into(), id: tt::TokenId::unspecified() })
     }
     fn symbol(&mut self, literal: &Self::Literal) -> String {
         literal.text.to_string()

crates/proc_macro_srv/src/abis/abi_1_56/rustc_server.rs

@@ -489,8 +489,8 @@ impl server::Literal for Rustc {
         // They must still be present to be ABI-compatible and work with upstream proc_macro.
         "".to_owned()
     }
-    fn from_str(&mut self, _s: &str) -> Result<Self::Literal, ()> {
-        unimplemented!()
+    fn from_str(&mut self, s: &str) -> Result<Self::Literal, ()> {
+        Ok(Literal { text: s.into(), id: tt::TokenId::unspecified() })
     }
     fn symbol(&mut self, literal: &Self::Literal) -> String {
         literal.text.to_string()